A laser printer printing process typically includes a series of steps for transferring an image from a photoreceptor to a page of paper moving through the printer. The steps include projecting an electrostatic charge from a primary charge roller onto the photoreceptor. A mirror moves in accord with laser printer scan data, such as a raster image, to direct a laser beam through a system of lenses and mirrors onto the photoreceptor surface. The photoreceptor surface containing the latent image is then exposed to toner. Next, the photoreceptor surface is pressed or rolled over paper, transferring the image onto the paper as it steadily moves through the printer. The paper then passes through a fuser assembly having rollers that provide heat and pressure to bond the toner to the paper. Finally, an electrically neutral rubber blade cleans any excess toner from the photoreceptor.
The laser printer printing steps are synchronized and the process requires that the paper continuously move through the printer. Therefore, unlike an ink-jet printing process which can be paused, if necessary, to obtain printer data, the laser printer process requires that the laser printer scan data be readily available so that it can be read at a continuous and uninterrupted pace from the start of the process until the page is complete.
Theoretically, the universal serial bus (USB) standard isochronous mode provides a rate of data transfer that is sufficient for a host based laser printer. However, the USB standard isochronous mode does not provide error correction and is therefore unsuitable for real-time transfer of laser printer scan data from a host computer to the laser printer's system of lenses and mirrors, commonly referred to as video hardware. To compensate for the lack of error correction during data transfer, current USB host based laser printer systems buffer a significant portion of a page at the printer before starting the printing process. The need to buffer a significant portion of a page places a heavy demand on laser printer memory. For example, a typical host based color laser printer buffers over 16 MB of data in the printer's RAM.